Audio Mirror vs Prima Luna tube DAC

Format is Redbook cd ripped to an external hard drive in .aiff format. Played back via iTunes into one of the 3 DACs hooked into the integrated amplifier. I use the Mac to select which DAC to use as playback device. I'm using USB input on all 3 of the DACs.  Both AM & PL claim to have good USB input. 
Any ideas?

Just for verification I hooked up my Rotel CD player which allows my to bypass its internal DAC using 75 ohm coax digital cable.  So now the comparison was Rotel DAC vs AM DAC vs PL DAC.  Same results.  Could not tell the difference between the DACs.

I've nothing terribly insightful, but do own a couple of Vlad's older DAC endeavors and they are quite good.  The new one should be dynamite.  I've listened to the Rega and it is competent, not particularly detailed or dynamic.

I guess I'm saying something in the chain may be amiss whilst not hearing discernible differences between these seemingly quite diverse products.

Currently run a high end R2R ladder DAC and have never looked back.

Congratulations on the AM pick.  It will reveal your smart choice upon break-in.

I’ve now demo’ed a Chord Qutest against my Rega. Same result. Qutest at best matches the Rega. Rega is slightly louder which most likely gives it the advantage.  There’s really not much in the chain to go wrong (unless iTunes Player is somehow affecting things - but that's why I brought in my Rotel CD player - to rule out iTunes). 

Rega no longer makes a DAC so considered Denafrips Ares or Pontus VS Schiit Bifrost or Gungnir.  Two supposedly well respected reviewers on another thread didn't have much good to say about Denafrips, so I ruled it out.  Bifrost 4 to 6 weeks out for shipping (I could see that extending in these times), so I pulled the trigger on the well reviewed Gungnir (aka: Gumby).

Really hope the Gumby betters the Rega, but I’m guessing it won’t...

Is there something wrong with the sound that you're getting now that's leading you to try all the different DACs?  

Seems like you've settled that there aren't differences in your system, so maybe the best route would be to keep buying cheaper and cheaper DACs until you find one that finally sounds worse.  Then keep the model right before that one, sell the others, and recoup your money.  

Or, if you really want to know what's possible in your system, just go ahead and get a new MSB Select.  If you can't hear that as different, then you can put the question to rest once and for all.